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Volume 2009, Article ID 475281, 14 pagesdoi:10.1155/2009/475281 Research Article Dynamic Resource Assignment and Cooperative Relaying in Cellular Networks: Concept and Performance Assess

Volume 2009, Article ID 475281, 14 pages

doi:10.1155/2009/475281

Research Article

Dynamic Resource Assignment and Cooperative Relaying in

Cellular Networks: Concept and Performance Assessment

Klaus Doppler,1Simone Redana,2Michał W ´odczak,3Peter Rost,4and Risto Wichman5

1 Radio Communication CTC, Nokia Research Center, It¨amerenkatu 11-13, 00180 Helsinki, Finland

2 Radio Systems, Research & Technology, Research, Technology and Platforms, Nokia Siemens Networks GmbH & Co KG,

St Martin Strasse 76, 81541 Munich, Germany

3 Applied Research, Telcordia Technologies, Telcordia Poland Sp z o.o., ul Umultowska 85, 61-614 Pozna´n, Poland

4 Vodafone Chair Mobile Communications Systems, Technical University of Denmark, Helmholtzstr 10, 01069 Dresden, Germany

5 Department of Signal Processing and Acoustics, Helsinki University of Technology, P.O Box 3000, 02015 TKK, Finland

Correspondence should be addressed to Klaus Doppler,klaus.doppler@nokia.com

Received 18 February 2009; Revised 19 May 2009; Accepted 1 July 2009

Recommended by Mischa Dohler

Relays are a cost-efficient way to extend or distribute high data rate coverage more evenly in next generation cellular networks This paper introduces a radio resource management solution based on dynamic and flexible resource assignment and cooperative relaying as key technologies to enhance the downlink performance of relay-based OFDMA cellular networks It is illustrated how the dynamic resource assignment is combined with beamforming in a macrocellular deployment and with soft-frequency reuse

in a metropolitan area deployment The cooperative relaying solution allows multiple radio access points to cooperatively serve mobile stations by combining their antennas and using the multiantenna techniques available in the system The proposed schemes are compared to BS only deployments in test scenarios, which have been defined in the WINNER project to be representative for next generation networks The test scenarios are well defined and motivated and can serve as reference scenarios in standardisation and research The results show that the proposed schemes increase the average cell throughput and more importantly the number

of users with low throughput is greatly reduced

Copyright © 2009 Klaus Doppler et al This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited

1 Introduction

Mobile users in next generation communication systems

are expecting seamless coverage with a guaranteed Quality

of Service (QoS) to allow for a similar user experience as

provided by today’s broadband internet connections This

causes a high spectrum demand of approximately 100 MHz

to support high aggregate data rates of up to 1 Gbit/s, which

will only be available at frequencies higher than 2 GHz The

World Radio Conference 2007 has, for example, identified

200 MHz at 3.4 GHz for IMT systems The high bandwidth

and carrier frequencies together with regulatory constraints

on the transmission power will limit the range for broadband

services Thus, many small cells are required for contiguous

coverage of areas with high traffic density

In-band relays are seen as a cost efficient way to extend

the high throughput coverage of next generation mobile

networks In [1] it was shown that deployments based on in-band relays can increase the high bit rate coverage at the cell border; thereby providing the means to balance the capacity within the cell and increase the coverage area Relays as part

of infrastructure based networks are currently standardised

in the Technical Specification Group j (TSG j) of IEEE802.16 [2] and it is currently a study item in 3 GPP [3]

The main focus of this paper is on the performance gain in the downlink of cellular relay networks compared

to base station (BS) only deployments in test scenarios that are foreseen for next generation cellular networks We propose two key radio resource management techniques to exploit the full potential of relay enhanced cellular OFDMA networks: dynamic and flexible resource assignment in

a relay enhanced cell and cooperative relaying We have developed these techniques during five years (2003–2008)

of extensive research on cellular relay networks within

the European research project WINNER [4] The dynamic

resource assignment adapts to changing user and traffic

densities and it is flexible enough to be applicable to

deployment scenarios ranging from wide area deployments

to local area office deployments In particular we show

how to adapt the dynamic resource assignment to a wide

area deployment which utilizes a grid of beams at the

base station and to a metropolitan area network utilizing

soft-frequency reuse for interference coordination Our

cooperative relaying proposal allows the cooperating radio

access points (base station or relay station) to utilize any

multiantenna technique used by the system to jointly serve

users

We present numerical evaluation results on the

achiev-able downlink gains from dynamic resource assignment and

cooperative relaying compared to BS-based deployments

The numerical results show the final assessment results in

a wide area, a metropolitan area, and indoor test scenarios

The results are based on an extensive set of system level

simulations after several iterations and refinements during

the course of the last three years Next to the results we

describe and motivate the used relay deployments in a wide

area, metropolitan area, and an indoor test scenario We have

defined these relay test scenarios in WINNER and they have

been contributed to the guidelines by ITU-R for evaluating

candidate radio interface technologies for IMT-Advanced

[5]

The remainder of this paper is organized as follows In

Section 2we give an overview on related work InSection 3

we present the test scenarios for a metropolitan area

(Manhattan grid), a wide area (hexagonal grid), and a local

area (office environment) relay deployment In Section 4,

we outline the proposed dynamic resource assignment for

relay enhanced cells and illustrate its application to the

test scenarios Further, we discuss different flow control

mechanisms and introduce our cooperative relaying concept

as an add-on to single-path relaying Thereafter, we present

inSection 5the performance assessment results obtained by

system level simulations for the proposed dynamic resource

assignment and cooperative relaying in the aforementioned

test scenarios

2 Related Work

The main focus of this paper is on the downlink system

performance of a cellular relay network There is few related

work in this area and the results have been obtained with

very different assumptions, that is, they are typically not

directly comparable Some of the results where obtained for

relaying scenarios where the relay station (RS) transforms a

non-line-of-sight (NLOS) base station-mobile station

(BS-MS) link into two line-of-sight (LOS) BS-RS and RS-MS

links The BS-RS links can be planned in a cellular network

for stationary RSs and the probability of an LOS BS-RS link

is increased However, the MSs can be located anywhere in

the cell and the probability of LOS to the BS should be at

least the same or even higher than to the RS because the BS

is typically deployed higher than the RS Thus, in order to

enable a fair comparison the properties of the BS-MS and

RS-MS links should only depend on the deployment In addition these papers consider all the interfering links to be NLOS, that is, the resulting Signal-to-Interference and Noise Ratios (SINRs) for the RS-MS links are too high In our studies we did not make such assumptions to ensure a fair comparison The downlink performance of a multicell WINNER network in a wide area scenario has also been studied in [6] Under the assumption of an LOS BS-RS and RS-MS link and NLOS BS-MS and interfering links the saturated throughput of the relay deployment is 25% higher in the relay deployment compared to the same deployment without relays However, this paper does not apply the dynamic resource assignment proposed in this paper and thus higher gains are expected under these assumptions

The IEEE 802.16j has issued a draft standard [7] and first performance results for the downlink of such a system are available In [8] a scenario with 14 RS added to each BS

in a macrocellular deployment with a cell radius of 1 km is studied Again an RS transforms an NLOS BS-MS link into two LOS BS-RS and RS-MS links Under this assumption the relay deployment increases the downlink capacity of the cellular network by more than 100% The results in [9] indicate that for relays that do not extend the coverage area

of a BS (transparent relays in IEEE 802.16j) the performance gains are below 5% In [10] different reuse pattern and path selection rules have been studied The results show that a macrocellular relay deployment can serve up to 90% more users than a BS-based deployment However, this comparison does not consider sectors at the BS and shadow fading as well as fast fading is not modeled Further, the RS transmission power is only 3 dB less than the BS transmission power, which would not result in significant cost savings due

to the use of relays

Another set of assessment results for a WiMAX relay deployment in a metropolitan area is available in [11,12] Unfortunately, there is no comparison with a BS only deployment but the results show significant gains from using directive antennas In this work it is assumed that the BSs and RNs are deployed at street crossings with directional antennas covering the streets leading to the crossing In practical deployments it will be hard to deploy a radio access point at street crossings and therefore our work focuses on

a deployment in the streets which is also recommended by

3 GPP in [13] and similar to [11,12] we also utilize directive antennas (sectors) at the BS Secondly, the previous work

in the metropolitan area has focused on outdoor users in the street whereas we consider also users inside the building blocks that typically account for most of the traffic in a cellular network

In addition to multicell studies, several aspects of the cellular downlink of OFDMA systems have been studied for a single cell In [14] the OFDMA resource allocation for a single relay enhanced cell with multiple users and a maximum C/I scheduler is analyzed In these studies the relay deployment achieves 15% higher data throughput and the outage probability is reduced from 30% to 20% In [15]

it is shown that the optimization of the subframe duration (RS transmits to MS/RS receives from BS) together with

RLC

MAC

PHY

RRC RLC MAC

RRC RLC MAC

Figure 1: RSs within the cellular network, the control plane

subcarrier allocation improves the overall cell throughput

compared to subcarrier allocation only as proposed in [16]

These single cell results confirm that the subframe duration

should be flexible as proposed by our dynamic resource

assignment

System performance results of relay-based deployments

for the cellular uplink for the WINNER system can be found

in [17] and for IEEE 802.16j in [18] Early performance

assessment results for cellular relay networks that are not

based on OFDMA can be found, for example, in [19] for the

integrated Cellular Ad hoc Relay System, in [20] for mobile

relays, and in [21] for a 1xEVDO system enhanced by relays

The results presented in this paper are the final

assess-ment results of the relay-based system developed in

WIN-NER Phase II [22] We have presented parts of the concept

and early performance results in [23–27]

Differently to our wide area results in [23] these are the

first results that have been obtained in a dynamic scenario

and we compare the performance of a relay deployment

with dynamic resource sharing to a BS only deployment

In addition we utilize the connection-based scheduling flow

control scheme that we have presented in the context of

WiMAX in [24] The results in [26] have been obtained

for relays deployed above rooftop and with more relays per

sector Increasing the amount of relays increases the benefits

due to cooperative relaying but it also increases the costs of

the deployment

The metropolitan area results in [25] did not utilize

soft-frequency reuse for the BS only scenario and the power masks

have been updated for the relay scenario considered in this

paper Further, we utilize the interference aware scheduling

scheme designed for soft-frequency reuse that we evaluated

for a BS only deployment in [28] This is also the first time

that we present results for outdoor users and show the effect

of a simple flow control on the system performance

The local area results in [27] compared different relay

deployment options whereas now we compare the expected

user throughput of a relay deployment to a BS only

deployment

3 Relay Properties and Test Scenarios

The design of a radio resource management scheme for

relay-based systems depends on the properties of the relays and on

the deployment of the relays In addition the multiantenna

techniques utilized in the system have to be taken into

account Therefore we introduce and motivate first the main

properties of the relays and the relay deployments considered

in our work The main motivation to deploy relays is to save costs while reaching a similar performance as less dense

BS only deployments or to increase the performance of

a BS deployment cost efficiently by adding relays Hence, most of the following design choices are motivated by cost considerations

In our test scenarios we allow an intelligent deployment with favorable propagation conditions between the base station (BS) and the relay station (RS), for example, line-of-sight (LOS) to the BS As a consequence the quality

of the BS-RS link can be very different from the RS-MS link Therefore, we consider only decode-and-forward relays (operating up to OSI layer 3), which can take advantage of dynamic resource allocation and adaptive transmissions with different modulation and coding schemes when receiving and forwarding data

The intelligent deployment assumption is based on cost comparison studies of relay based and BS only deployments For intelligent relay deployments studied in [29, 30] RSs are already cost efficient if the costs are 88% of the costs

of a micro-BS Without intelligent deployment the RS cost should be only 6.5% of the BS costs [31]

The number of RSs per BS is an important design parameter that affects both the costs and the performance

of the relay network We have limited the number of RSs to three per BS sector based on the result curves in [29] which

do not suggest more than 4 RSs per BS in a scenario similar

to the one considered in our work

To keep the size of RSs small we assume in all scenarios

a limited transmit power for RSs and a maximum of two antennas Small RSs that do not require shelter, cooling, and backhaul connection increase the deployment flexibility and allow, for example, a deployment on lamp posts Thereby the site acquisition and site rental costs can be reduced even compared to a micro- or pico-BS According to cost studies in [32] site rental and the cost of the transmission line account for more than 60% of the overall costs of a micro-BS over 10 years

Finally, we require that adding a RS to the network does not increase the cost of an MS This is achieved by

a RS that provides an identical interface towards an MS

as a BS, that is, the MS does not need to distinguish between RS and BS and both are referred to as radio access points Further, we focus on in-band relays that do not require additional bandwidth The resulting multihop cellular system architecture is illustrated inFigure 1[33] A Relay Enhanced Cell (REC) is formed out of a BS together with its associated RSs

Our test scenarios are primarily designed and optimized for two hops (BS-RS-MS) in order to achieve a high performance in terms of throughput and delay Further, we assume a tree topology to avoid the overhead from complex routing protocols In the rare case of node failure the RS can autonomously connect itself to another radio access point in its range

For base station-based deployments the hexagonal grid cell layout with variable intersite distance and the Manhattan grid following the UMTS 30.03 recommendations [13] have

been accepted as evaluation scenarios in standardization and

research However, no such widely accepted scenarios exist

for relay deployments and the results of different research

groups are not comparable

In the following we present relay test scenarios for

three typical deployments of future wireless communication

systems: a wide area scenario that provides base urban

coverage based on a hexagonal cell layout, a metropolitan

area scenario based on microcells deployed in a Manhattan

grid, and a local area office scenario Both the wide area

and the metropolitan area scenarios cover the important

case of an operator that wants to upgrade an existing

UMTS network and to reuse the existing BS locations The

properties of the MS are the same in all scenarios (seeTable 4

inAppendix A)

All three test scenarios use path loss and channel

models developed in Phase II of the WINNER project The

properties of the channel model and a comparison to other

models can be found in [34] The path-loss equations and

the corresponding channel models can also be found in [35]

Since [35] offers several possible path-loss models for each

link type we state the path-loss equations used in the test

scenarios in AppendicesB,C, andD

3.1 Wide Area Test Scenario The wide area test scenario is

an urban macrocellular deployment It aims at providing

ubiquitous coverage in an urban environment resulting in

rather large cells, having a radius up to several kilometers

Base stations (BSs) are consequently expected to provide

high power outputs, in each of the three sectors, equipped

with four antennas All BSs are deployed above rooftop

(hBS = 25 m), possibly requiring additional masts for

their installation This implies that the site selection and

rental costs will probably be dominant with respect to the

other costs such as the backhaul infrastructure We further

consider RSs which are deployed below rooftop (hRS =5 m)

with a single antenna and a significantly lower output power

in order to keep costs low and allow for a flexible deployment

of multiple RSs per sector For the same reason the RS is

equipped with a single antenna

We distinguish between a carefully planned RS

deploy-ment with a high probability of line-of-sight (LOS) and a not

carefully planned RS deployment without LOS to the BS For

both cases and the BS-MS link we assume an urban

macro-cell model whereas we assume for the RS-MS a non-LOS

(NLOS) microcell model (seeAppendix Bfor details)

The cells form a regular grid with a hexagonal layout

and an intersite distance (ISD) of 1000 meters We study

this scenario with three RSs per sector according to the

deployment in Table 1 It provides, as shown in Figure 2,

a good coverage for MSs at the cell border Moreover, we

consider also the scenario with only one RS per sector (also

shown inFigure 2) for comparison purpose The exact RS

deployments are outlined inTable 1for both scenarios

3.2 Metropolitan Area Test Scenario The metropolitan area

test scenario is an urban micro-cellular scenario modeled

by a two-dimensional Manhattan grid consisting of 12×12

−100

0 100 200 300 400 500

x (m)

BS

RN

θ

(a) 1RS per sector

−100

0 100 200 300 400 500

x (m)

BS

RN

RN

RN

θ

(b) 3RS per sector Figure 2: Coverage area for BS and RS in wide area test scenario

Table 1: RS deployment in the wide area test scenario

per sector distance (m) (relative to sector broadside direction)

streets (width 30 m) and 11×11 buildings (200 m×200 m block size) The BS deployment follows the UMTS 30.03 recommendation [13] with 73 BS deployed below rooftop level (10 m height) and placed in the midpoint between two crossroads Two sectors are formed with bore-sight along the street direction and one antenna per sector The added relays extend the coverage area of these BSs and distribute the cell capacity more evenly

Power masks [1 0.5 0.25]

[0.5 0.25 1]

[0.25 1 0.5]

Figure 3: Sketch of metropolitan area cell layout with relay stations

and assigned soft-frequency reuse power masks

A single RS (5 m height) is added to each BS site in the

midpoint between two BSs, as depicted inFigure 3 Thereby

the number of radio access points is doubled Adding a

second relay per BS site increases the cell throughput only

slightly and does not justify the additional costs [25] The

RSs are equipped with two antennas, a directional antenna

to communicate with the serving BS and an omnidirectional

antenna to serve its MSs The power masks assigned to

each BS and RS in Figure 3 are used by an interference

coordination scheme based on soft-frequency reuse which is

described inSection 4 The transmit power of the RS is 7 dB

lower than the transmit power of the BS to enable a smaller

physical size

A LOS link is assumed for nodes in the same street and a

NLOS link for nodes in different streets and MSs are located

inside a building or in a street Details of the propagation

model and additional simulation parameters can be found in

Appendix C

3.3 Local Area Test Scenario The local area test scenario

is defined as an isolated hot-spot-like indoor area with

high user density where the users are either stationary or

slowly moving It is characterized by high shadowing and

considerable signal attenuation due to the existence of rooms

separated by walls As a result of the isolated characteristics

the interference is much lower compared to the previous two

scenarios The scenario consists of one floor (3 m high) in

10 20 30 40

Figure 4: Local area scenario with two BSs (dark gray) and four relay stations (light gray) to assist each BS

a building with two corridors (5 m×100 m) and 40 rooms (10 m×10 m)

A deployment with two single antenna BSs (dark gray nodes) is presented in Figure 4 They are located in the middle of the corridors, halfway from the left/right side of the building Each of them is assisted by four single antenna RSs (light gray nodes): two on the left and two on the right side, respectively (i.e., 10, 30, 70, and 90 meters from the left or the right side of the building) as depicted inFigure 4 All the area marked with gray color benefits from the use of (cooperating) RSs

A LOS or NLOS office propagation model is employed depending on the presence of walls between the BS, RSs, and MSs Details of the propagation model and additional simulation parameters can be found inAppendix D

4 Radio Resource Management in Relay Enhanced Cells

Relays add additional degrees of freedom to the radio resource management of a cellular system The RS can act as a BS to serve its MS or as an MS to receive data from the BS The coverage area of an RS is lower than for a BS due to the lower transmit power and different deployments Nevertheless, they should be integrated and evaluated together with the interference coordination and multiantenna techniques utilized in the network On the other hand the cooperation of multiple radio access points

is easier in a relay enhanced cell than between BSs since the

BS can act as a coordinating node in the resource allocation for cooperatively served users

In the following we propose the following radio resource management techniques for relay enhanced cells: dynamic resource assignment, flow control for multihop connections and cooperative relaying as an add-on to single-path relay-ing

4.1 Dynamic Resource Assignment A fixed and static

resource assignment will not allow to exploit the full poten-tial of relay-based deployments since the relay deployments can have very different properties as illustrated inSection 3 Therefore, we propose that the BS flexibly assigns parts or all of the available system resources to itself and to each RS

in the relay enhanced cell In particular the BS assigns the

frames in which the RS communicates with the BS (act as

an MS) or serves its MS (act as a BS) Further, it assigns the

OFDMA resource units (chunks) that the RS can use in the

frames for which it acts as a BS The assigned resources are

then available for autonomous scheduling at each individual

radio access point Figure 5illustrates an example resource

allocation for a BS with three RS in its cell

The actual resource assignment strategy depends on

the utilized interference coordination and multiantenna

techniques In the wide area test scenario beamforming has

been shown to be an effective way to improve the cell capacity

[37] We propose to coordinate the interference from the

subcells formed by the BS to the subcell formed by RSs

by using at the BS beams with low interference to the RS

subcell for resources that have been assigned to the RS

The amount of resources for the RS is dynamically adjusted

depending on the traffic and interference situation We refer

to this approach as Dynamic Resource Sharing (DRS) [38]

DRS uses logical beams which can be seen as a dynamic

version of sectors The Dynamic Resource Sharing (DRS)

acts in three steps: the creation of the beams, grouping of

the beams, and the actual resource assignment [38] For

the assessment results presented inSection 5we utilize the

resource assignment that we proposed in [23] which aims to

achieve the maximum possible cell throughput by allocating

an OFDMA resource unit (chunk) to the group of beams that

can reach the highest total rate

In the metropolitan area scenario we study an

interfer-ence coordination scheme based on soft frequency reuse

It assigns power masks (in the frequency domain) to

neighboring radio access points to coordinate the mutual

interference Thereby, soft frequency reuse enables frequency

reuse one and at the same time each radio access point has

high power resources with reduced interference available to

schedule MS located at the border area Soft frequency reuse

is better suited for the metropolitan area than beamforming

because the radio signal propagates very well in the street

canyons making it difficult to separate different beams

Further, interference coordination is mainly needed at street

crossings and in the border area between radio access points,

whereas the border area is smaller than in a wide area

deployment

In the local area scenario we make use of the fact that

the BSs and RSs located in different corridors are separated

by at least three walls which can be perceived as a natural

means of suppressing interference Due to the physical

separation, sharing of the same resources may be possible for

multiple transmissions In cases where it is not possible to

share the resources, the users are either served cooperatively

by multiple radio access points or exclusive resources are

assigned

Table 2summarizes the essential elements of the resource

assignment The MS does not need to perform additional

measurements to support the resource assignment The BS

uses the received signal strength from neighboring radio

access points (BS or RS) reported by the MS as an input,

which are anyway required for handover purposes Please

note that the logical beams are a dynamic version of sectors

Table 2: Example of essential elements of resource assignment scheme

Resources to be assigned

Frames in superframe where

RS serves MS/communicates

to BS, chunks assigned to RS, power mask to be used for chunks

Granularity of resources

Group of four OFDMA resource units (chunks) in the frequency domain,

TDMA frame in the time domain (0.7 ms) Measurements/information related

Measurements required

Received signal strength of neighboring radio access point sector (beam) Who performs measurements MS

Additional information Estimate of required chunks

to serve MS

message every 100 ms Who collects it Serving radio access point

Resource assignment message

Content

Power mask (MA), frames assigned to serve MS in superframe, chunks assigned within the UL/DL frames to the RS

and therefore also measurements for the logical beams will

be available

Real world deployments are not as regular as the presented test scenarios and due to the small size of the subcells formed by BSs and its RSs the traffic density can vary significantly in these subcells The proposed dynamic resource assignment scheme offers sufficient flexibility to adapt better to real world situations than a static resource assignment

4.2 Flow Control In WINNER we propose a distributed

scheduling, that is, the BS assigns resources to itself and the RSs in the relay enhanced cells but it does not centrally schedule the transmissions to the MSs The RSs can then independently allocate these resources to its associated MSs Thus, frequency adaptive transmissions and multiantenna transmission schemes can be supported without forwarding channel state information, precoding weight feedback, and

so forth to the BS This decision can be justified by the results in [14, 15] which indicate a performance loss of less than 10% compared to a centralized scheduler even without considering the signaling overhead for a centralized scheduler

However, when utilizing distributed scheduling the BS should be aware of the buffer status of each MS or flow at the RS If it forwards too much data to the RS eventually the buffer of the RS will overflow and if it forwards too

F P

BS Rx RN1 Rx RN2 Rx RN3 Rx

BS Tx RN1 Rx RN2 Rx RN3 Rx

BS Rx RN1 Tx RN2 Tx RN3 Rx

BS Tx RN1 Tx RN3 Rx

BS Tx RN3 Rx

RN2 Tx

BS Rx RN1 Rx RN2 Rx RN3 Rx

RN1 Rx RN2 Rx

BS Tx RN1 Tx RN2 Rx RN3 Tx

BS Rx RN1 Rx RN3 Tx

BS Rx RN2 Rx RN3 Tx

BS Tx RN1 Tx RN2 Tx RN3 Tx

BS Rx RN1 Rx RN2 Rx RN3 Rx

BS Tx RN1 Rx RN2 Tx RN3 Tx Payload = 8 × 0.6912 = 5.53 ms

Frame = 0.6912 ms

Time

.

.

RN1

RN2

RN3 BS

act as BS

RN1 act as BS RN2 act as BS RN3 act as BS

RN2 act as BS RN3 act as MS Figure 5: Example allocation of a superframe using the Flexible Resource Assignment scheme in a relay enhanced cell with three relays (RSs) The super-frame consists of a preamble and an 8-frame payload following the WINNER system specifications [36] The Base Station (BS) allocates (a part of) the resources to the RSs, the RSs independently schedule their associated MS within their allocations when acting

as BS

little data the MS will be starved Even if the buffer at

the RS is large enough to store all the data for the MS,

the resources on the BS-RS link have been wasted when

the MS performs a handover to another RS or BS In

our work we have considered two different approaches to

flow control: connection-based scheduling (CbS) and

stop-and-go signaling The results in Section 5 show that both

schemes are well suited for the considered deployments with

a maximum of two hops

The CbS is a resource request and allocation strategy

proposed in [24] for controlling the resources and delays of

multihop communications with different numbers of hops

Each RS requests to the BS not only the needed resources

for data transmission on the access link between the RS

and the MSs but also on the multihop links from/to the

BS Every RS computes the resources required for each

end-to-end connection served by the RS instead of only

the next link towards the destination The BS collects the

resource requests and grants resources on each hop for each

connection (uplink and/or downlink) between the BS and

each RS

The stop-and-go flow control requires less signaling than

the CbS but CbS is better suited for deployments with more

than two hops It depends on the rate of the RS-MS link The

RS sends a stop signal for an MS to the BS when the queue

size for the MS exceedsι The queue size ι depends on the

current channel quality of the RS-MS link and is calculated as

whereRfullBWdenotes the predicted rate (based on channel quality feedback) when the MS is assigned the full bandwidth andn is a parameter that can depend on the number of users

served by the RS and the amount of frames where the RS serves its MSs For the numerical assessment results in the metropolitan area we have used a fixed parametern =2 and compare the performance of the proposed flow control to a scenario without flow control

4.3 Cooperative Relaying as Add-On to Single-Path Relay-ing Next to the flexible resource assignment, we propose

cooperative relaying to further enhance the capacity of a relay enhanced cell In the DL of single-path relaying, the data is first transmitted from the BS to the RS and then the RS forwards this data to the MS (We refer in the

following to noncooperative relaying as single-path relaying,

because only a single transmission path between source and destination is exploited.) To gain on large-scale spatial diversity, most cooperative relaying protocols proposed in

literature, for example, [39–41] benefit from a combination

of the transmissions in two phases, first from the BS and

then from the RS An overview and classification of different

cooperative relaying protocols can be found in [42–44]

As the transmission from a BS is received by the MS and

the RS, dedicated multiantenna techniques (beamforming

and other space division multiple access (SDMA)

algo-rithms) can be applied only partially, because one stream

is only optimized for one destination Furthermore, as we

assume an intelligent deployment, the achievable data rate on

the BS-RS link is likely to exceed the data rate on the RS-MS

links However, to enable cooperation on the physical layer

the same modulation and coding scheme or only a limited

set of specialized and sophisticated modulation and coding

schemes can be used

Thus, we do not only consider cooperative relaying that

exploits large-scale spatial diversity but we investigate mainly

cooperative relaying, where multiple radio access points form

a Virtual Antenna Array (VAA) [45] Any multiantenna

transmission technique, including spatial multiplexing, can

then be applied, for example, to the BS antennas augmented

by the antennas of an RS In Section 5 we present results

for a cooperative multiuser MIMO relaying scheme that we

proposed in [26] It utilizes distributed LQ precoding which

has been introduced for cooperating BSs in [46] and a dirty

paper coding technique as proposed in [47]

In our cooperative relaying proposal the first common

node in the tree topology schedules the cooperative

trans-mission Thus, in a network that is limited to two hops, the

BS allocates resources to all cooperative transmissions in a

similar way as in single hop networks using similar feedback

information The BS then sends the resource allocation

and the selected transmission mode (MIMO transmission

scheme, precoding weights, modulation and coding scheme

for different streams, etc.) together with the data to the

RS(s) Both BS-RS cooperation and RS-RS cooperation are

supported.Figure 6illustrates restrictions at the RS resulting

from cooperatively served MSs The RS has to take these

restrictions into account when allocating resources to the

MSs served solely by the RS within the resources assigned

from the BS

When calculating the precoding weights for a

cooper-ative (multiuser) MIMO transmission scheme the channel

matrices of all the cooperating nodes have to be forwarded to

the BS and the precoding weights have to be transmitted to

the RS before the cooperative transmission Due to this high

amount of data which has to be communicated between BS

and RS(s), MIMO cooperative relaying is more affected by a

limited BS-RS link capacity than single path relaying Hence,

the proposed MIMO cooperative relaying solution requires a

high capacity BS-RS link which can be guaranteed by a

line-of-sight assumption between BS and RSs

The highest gain from cooperative relaying is obtained

if the signals received from the cooperating radio access

points are of similar strength Therefore we base the decision

which radio access points (BS or RS) should form the VAA

on the received signal strength reported by the MS and

RS In particular we propose the use of a static version

of the REACT algorithm [48] The original algorithm was

Time

Chunk

Assigned to RS Assigned to cooperative transmission

Cooperative transmission to MS1

Cooperative transmission to MS1 Cooperative transmission to MS2

Figure 6: Scheduling restrictions at the RS The RS receives resource allocation for cooperative transmissions from the BS Together with the flexible resource assignment this restricts the resources the RS can use to schedule noncooperative transmissions

developed for mobile ad hoc networks with relays and due

to the fact that in the scenario under investigation the RSs are located at fixed positions there is no need to perform periodic neighbor discovery and topology recognition The static version of REACT is executed by the BS and exploits information about power levels of the signals received by MSs from different radio access points (BS or RS) as well as the power levels of the signals received by RSs from the BS Thus, the BS has a good overview of the topology to select the cooperation type

Next to data transmissions the MSs have to receive control information In our cooperative relaying proposal the control information is not transmitted cooperatively but each MS has a serving RAP which can be the BS or an RS

In either case, the serving RAP performs retransmissions, transmits the broadcast channel, receives feedback from the

MS, and signals the resource allocation to the MS

4.4 Applicability to IEEE802.16j The IEEE802.16j draft

standard [49] allows already a dynamic resource assignment

in the time domain by adjusting the duration of the relay zone but no mechanism has been standardized for the frequency domain In the case of dynamic resource sharing the resource assignment in frequency domain can simply

be done by signaling chunks (subchannels in WiMAX terminology) that should not be used by an RS For soft-frequency reuse, in addition the power mask to be applied for chunks has to be signaled Thus, with small additional

signaling the 802.16j standard can support the flexible

resource assignment proposed in this paper

The draft 802.16j standard [49] also specifies the

pos-sibility for cooperative BS-RS transmissions It mentions

two basic possibilities: cooperative source diversity

(repeti-tion coding) and cooperative transmit diversity established

through distributed space-time block coding (STBC) and

a combination of both We propose a much more flexible

scheme that supports also RS-RS cooperation and any

MIMO scheme that is used in a system Thus, major

additions would be required to the standard in order to

support our concept

5 Numerical Results

In this section we present performance assessment results

for the dynamic and flexible resource assignment and

cooperative relaying in a multicell OFDMA network We

compare the performance of relay deployments to BS only

deployments in the test scenarios presented inSection 3 For

the metropolitan area and the cooperative relaying results in

the wide area we assume two antennas at the RS and a single

antenna otherwise

All results have been obtained in system level simulations

using the link to system level mapping of [50] and parameters

from the WINNER system.Table 5inAppendix Apresents

the main parameters of the FDD physical layer mode utilized

for the wide area assessment of DRS and the TDD physical

layer mode of the WINNER system which has been used in all

other scenarios For both modes an overall system bandwidth

of 100 MHz was chosen in order to meet the peak data rates

that were established as research targets for systems beyond

IMT-2000 [51]

All simulations have been performed with a full buffer

traffic model and the MSs are selected for scheduling at

the BS and the RS by a round robin scheduler In the

metropolitan area we additionally utilize the channel aware

scheduling in the frequency domain that we proposed in

[28] The MSs are associated with the strongest radio access

point (BS or RS) in the case of single-path relaying In the

case of cooperative relaying they are jointly served by BS and

RS if the received signal power of the two radio access points

is within 20 dB RS-RS cooperation is not considered in this

scenario since the RSs do not have large overlapping coverage

area

The results have been obtained for the center cell in

the wide area scenario and for two center cells in the

metropolitan area In both cases the center cells were

surrounded by 2 tiers of interfering cells In the metropolitan

area, the radio access points (BS and RS) have been divided

into three groups and a relative power level pattern has

been assigned to each group, as illustrated inFigure 3 The

absolute power levels depend on the maximum transmit

power of the radio access point The power mask levels

have not been optimized but we believe they are reasonable

choices

The results inTable 3compare the average cell

through-put and the fifth percentile of the user throughthrough-put

cumula-tive distribution of a BS only deployment to a relay-based

deployment in the wide area and metropolitan area test scenario with different radio resource management options

5.1 Dynamic Resource Allocation in Wide Area Test Scenario.

In this analysis the deployment positions of RSs are not optimized with respect to the propagation conditions to the

BS Therefore an NLOS model is assumed and the path-loss between BS-RS is calculated as in (B.2)

The wide area results on DRS in Table 3show that the DRS outperforms the BS only deployment By utilizing this approach the cell throughput is increased by 25% with only one RS per sector and by almost 50% assuming 3 RSs per sector.Table 3 also shows results for a Fixed Resource Partitioning (FRP) without coordinating the beams at the

BS with RS transmissions The static resource partitioning

is based on the following considerations The relay coverage area is about one forth of the sector area, as shown

in Figure 2 The throughput of the relay link (BS-RS) is assumed to be twice the average throughput of the RS-MS links Further, the throughput per user in the coverage area

of a BS is assumed to be the same as in the coverage area of

an RS To avoid interference the BS does not serve its MSs while the RS serves its MSs Hence, the resource demand for the different links was estimated to be 6/9 for the

BS-MS links, 1/9 for the BS-RS links, and 2/9 for the RS-MS

links With this static resource partitioning we can observe that the average cell throughput is reduced by 30% compared

to the BS only scenario Thus, without properly assigning the resources inside the cell the potential benefits of relaying are lost and the performance might even degrade

5.2 Soft Frequency Reuse in Metropolitan Area Test Scenario.

In the metropolitan area we compare the performance of

a relay deployment using the flexible resource assignment proposed in Section 4and soft frequency reuse (SFR) to a

BS only deployment These studies assume a slowly changing resource assignment for the studied part of the network which remains constant during the simulated 70 seconds

of network operation This models a flexible resource assignment that adapts to slow variations, for example, depending on the time of the day, and the same assignment

is used for all cells in this part of the network

Table 3shows the results both for users located indoors and in the streets The outdoor to indoor coverage of the BS only deployment is limited and adding relays is especially beneficial for users with low throughput in the BS only deployment As a result the fifth percentile of the user throughput CDF more than quadruples However, for users

in the street the BS only scenario is already interference limited and adding RSs does neither increase the cell throughput nor the fifth percentile of the user throughput CDF

We allow both the RS and BS to serve its MSs at the same time which achieves significantly better results compared to

BS and RS serving MSs in separate frames The amount of frames within a superframe where the RS is serving MSs depends on the capacity of the BS-RS link and the RS-MS links As the capacity of the BS-RS link is very high, the best

Table 3: Relative performance of BS only and RS deployment with different resource assignment options in the test scenarios.

Wide area DRS

Metropolitan area indoor SFR

Metropolitan area outdoor SFR

result was achieved when the RS serves its MSs in five out of

eight frames Thus, three out of eight frames are sufficient for

the BS-RS communication Selecting the optimal number of

frames for the RS transmission improves the fifth percentile

of the user throughput CDF by 38% and the average cell

throughput by 4% compared to an assignment where the RS

serves its MSs in every other frame This indicates that the

performance of relay deployments strongly depends on the

proper balance between the resources spent on the first hop,

between BS and RS, and on the second hop, between RS and

MS

We also studied the impact of flow control on the overall

performance of the network For the case without flow

control we set the stop limit to 25 Mbit per flow which

corresponds to about 8 seconds of data for an MS Without

flow control the average cell throughput decreases by less

than 1% and the fifth percentile of the user throughput CDF

by 3% The impact of flow control is rather limited in this

scenario since the BSs transmit data to the RSs only in 38%

of the frames and RSs are only present in every second sector

The conclusions will likely be different in a scenario with

more relays and more than two hops Especially for more

than two hops a flow control based on connection-based

scheduling is likely the better option

5.3 Cooperative Relaying in Wide Area Test Scenario

Coop-erative relaying can further enhance the performance of

a relay deployment To evaluate the potential benefits of

cooperative relaying we compare the cooperative multiuser

MIMO relaying scheme with single-path relaying and a

system using only direct links between BSs and MSs (BS

only) The path loss for the BS-RS link assumes a careful relay deployment and is calculated as in (B.3)

Figure 7presents the CDF of the expected user through-put Θ(·,·) We can clearly observe from the CDF of the throughput that the number of users with low throughput

is significantly reduced, compared to a system without relay stations Besides, we can observe a major performance advantage of cooperative relaying in comparison to single-path relaying This is of course at the cost of additional sig-naling and control overhead Nonetheless, the coordinated and joint transmission of BSs and RSs seems to be a viable option especially in those areas where an MS experiences similar channel conditions to both radio access points

5.4 Cooperative Relaying in Local Area Test Scenario In

the local area test scenario, we assess the performance of cooperative relaying for the deployment given in Figure 4

[27] We compare two different possibilities The MSs are served by the BS or by RS using either single-path relaying (BS-RS-MS) or cooperative relaying (BS-VAA-MS), where

a Virtual Antenna Array (VAA) is formed by a pair of cooperating RSs The RSs forming the VAA are chosen with the use of a static version of the REACT algorithm as described inSection 4.3

The results were obtained for a fixed modulation and coding scheme based on QPSK modulation and the (4, 5, 7) convolutional code with the use of the fixed resource assignment in [27] Further, an AWGN channel model was assumed The presence of an outdoor network is modeled by setting an average interference power level of−125 dBm per subcarrier